Method and system for determining data transfer margins for network connections

ABSTRACT

Method and system for determining data transfer margins and/or bit rates for network connections, the physical length of a network connection between a transmitter and receiver being known. A power spectrum is measured depending upon transfer frequency for possible modem types. Attenuation, and thus actual signal strength, is determined for different physical lengths and cable wire thicknesses of the network connection. Noise level is determined at least depending upon crosstalk parameters, number of interference sources, and equalizer factors based on the power spectrum. The data transfer margins are determined for different data transmission modulations and/or modulation codings for a pre-defined bit rate, based on the actual signal strengths and corresponding noise levels, by a Gaussian transformation module. The data transfer margins are corrected by a correction factor containing average deviation of the stored data transfer margin with respect to the actual data transfer margin. The data transfer margin for the network connection is determined based on the stored actual data transfer margins with reference to the known physical length.

The present invention relates to a method and system for determiningdata transfer margins for network connections, in which method andsystem the physical length of a network connection between a transmitterand a receiver is known. In particular the method relates to networksbased on copper wire connections.

Traditional telephone network services, also called POTS (Plain OldTelephone Service), usually connect households and smaller enterprisesto a distribution station of the telephone network operator via copperwires which are wrapped around each other and are called twisted pairs.These were originally intended for ensuring analog signals, inparticular sound and voice transmissions. These requirements havehowever changed, at the latest with the emergence of the Internet andthe data flow connected therewith, and are rapidly changing once againtoday, owing to the need to be able to work at home and/or in the officewith real time and multimedia applications.

Data networks, such as e.g. Intranet and Internet, rely heavily onso-called shared media, i.e. on packet-oriented LAN (Local Area Network)or WAN (Wide Area Network) technologies both for broadband backbonebetween switches and gates and for local network connections withsmaller bandwidths. Use of packet manager systems, such as e.g. bridgesor routers, are widespread for connecting the local LAN networks to theInternet. An Internet router must thereby be capable of transmittingpackets accordingly, based on the most varied protocols, such as e.g. IP(Internet Protocol), IPX (Internet Packet exchange), DECNET, AppleTALK,OSI (Open System Interconnection), SNA (IBM's Systems NetworkArchitecture) etc. The complexity of such networks, in order to be ableto distribute the packets worldwide, is a challenge both for the vendorof services (provider) and for the manufacturer of the necessaryhardware.

The ordinary LAN systems work relatively well with data transfer ratesof about 100 Mbps. With transfer rates above 100 Mbps, the resources ofthe network manager, such as packet switches, do not suffice in most oftoday's networks for administering the allocation of bandwidths and ofuser access. Of course the usefulness of packet-based networks fortransmission of digital information, in particular with short-termtransmission peaks, was recognized long ago. Such networks usually havepoint-to-point structure, a packet being transmitted from a singletransmitter to a single receiver in that each packet comprises at leastthe destination address. A typical example of this is the known headerof an IP data packet. The network reacts to the data packet by routingthe packet to the address of the assigned header. Packet-based networkscan also be used for transmitting data types requiring a continuous dataflow, such as e.g. sound and audio transmissions of high quality orvideo transmissions. The commercial use of networks makes itparticularly desirable for packet-based transmission to be also possiblesimultaneously to a plurality of end points. An example of this is theso-called packet broadcasting for transmission of video or audio data.So-called pay TV can thereby be achieved, i.e. broadcast transmission,liable to charges, of video data over the network.

With the next generation of applications, such as real-time andmultimedia applications with their much bigger requirement with respectto bandwidth, which must be guaranteed moreover at any time, thepacket-oriented networks meet their limits, however. Thus a nextgeneration of networks should possess the possibility of reconfiguringthe networks dynamically in order to be able to always guarantee theuser a predefined bandwidth for requested or agreed-upon QoS Parameters(Quality of Service). These QoS comprise e.g. access guarantee, accessperformance, fault tolerance, data security, etc. between all possibleend systems. New technologies, such as e.g. ATM (Asynchronous TransferMode), should help to create in the long-term development of thenetworks the necessary prerequisites for the private Intranet as well asthe public Internet. These technologies promise a more economical andmore scalable solution for such high performance connections guaranteedby means of QoS parameters.

One change for future systems will also relate in particular to the dataflow. The data flow today is usually based on a server-client model,i.e. data are transmitted from many clients to or from one or morenetwork servers. The clients create normally no direct data connection,but instead they communicate with each other via network servers. Thistype of connection will also continue to have its significance.Nevertheless it is to be expected that the quantity of data which istransmitted peer-to-peer will increase sharply in the future since, inorder to meet the demands, the ultimate goal of the networks will be atruly decentralized structure in which all systems are able to act bothas server and as client. Thus the network will have to generate moredirect connections to the different peers, whereby e.g. desktopcomputers will be connected directly via the backbone Internet.

It is therefore clear that in future applications it will become moreand more important for the user to be able to be guaranteedpredeterminable QoS parameters and large bandwidths.

Used for data transmission to the end user are in particular thetraditional public telephone network (PSTN: Public Switched TelephoneNetwork) and/or PLMN (Public Land Mobile Network), which were actuallydesigned originally for pure sound transmission, and not fortransmission of such quantities of digital data. The so-called “lastmile” plays a decisive role thereby in the determination of the QoSparameters which a provider or vendor of telephone services is able toguarantee the user. Designated as the last mile is the stretch betweenthe last distribution station of the public telephone network and theend user. In the fewest cases the last mile consists of high-capacityfiber optic cables. It is usually based rather on the ordinary copperwire cabling, such as e.g. cable with 0.4 or 0.6 mm wire diameter. Thecables moreover are not run everywhere underground in protected groundconducting construction, but also consist of overland lines to telephonemasts, among other things. Additional disturbances thereby arise.

A further problem in determining the maximal QoS parameters is theso-called crosstalk problem. This problem arises with the modulation ofthe signal on the line e.g. from the end user to the distributionstation of the telephone network operator and vice-versa. Known in thestate of the art for modulation of digital signals are e.g. the xDSLtechnologies (Digital Subscriber Line), such as ADSL (Asymmetric DigitalSubscriber Line), SDSL (Symmetric Digital Subscriber Line), HDSL(High-data-rate DSL) or VDSL (Very high speed Digital Subscriber Line).The mentioned crosstalk is the physical phenomenon which arises duringthe modulation of data via a copper cable. By way of electromagneticinteraction, adjacent copper wires inside a copper cable obtain partialsignals pairwise which are generated by the modem. This results in xDSLmodems, carried on adjacent wires, interfering with one another. Adistinction is made between Near End Crosstalk (Next), whichcharacterizes the undesired signal coupling of signals of thetransmitter at one end to the signals of the receiver at the same end,and Far End Crosstalk (FEXT), which characterizes the undesired signalcoupling of signals during the transmission to the receiver at the otherend, the signals during the transmission being coupled to signals ofadjacent copper wire pairs and showing up as noise at the receiver.

Although many studies on xDSL crosstalk are available today, such ase.g. “Spectral management on metallic access networks; Part 1:Definitions and signal library”, ETSI (European TelecommunicationsStandards Institute), TR 101 830, September 2000, there are at thepresent time few usable, technically easy-to-handle and cost-efficientaids for determining the QoS parameters for a particular end user in thenetwork, owing to the complexity of the crosstalk phenomenon and of theremaining noise parameters. In the state of the art, remote measuringsystems have been proposed by various companies, such as e.g. Acterna(WG SLK-11/12/22, Eningen, among others, Germany), Trend Communications(LT2000 Line Tester, www.trendcomms.com, Buckinghamshire, U.K.) etc. Themaximal transfer rate over the last mile is thereby determined throughdirect measurements by means of remote measuring systems: a digitalsignal processor is installed at each local distribution station of atelephone network operator (e.g. in Switzerland several thousand). Bymeans of the digital signal processor a so-called “single endedmeasurement” is carried out since no installations of devices arenecessary at the user on the other side of the last mile. Themeasurements are also possible, in principle, by means of “double endedmeasurement.” Installation of measuring devices at both ends of the lineare thereby necessary, however.

The drawbacks of the state of the art are, among other things, highcosts from the required installation of remote measuring systems atevery local distribution station and a imprecisely known incertitude orrespectively unknown error during the measurement since the measurementsare carried out only on one side (single ended) and measurements on bothsides would be needed to determine the error. A two-sided measurementwould not be feasible in view of the investment in personnel and in timeas well the in costs. Also lacking in the state of the art arealgorithms with their hardware or software implementation forcalculation, or respectively prediction, of the maximal possible bitrates of a network connection. An installation of the remote measuringsystems at the less numerous central distribution stations instead of atthe local end distribution stations shows that the measurements entailsuch great uncertainties that they are not suitable for determining themaximal possible data throughput rates for a particular line to an enduser.

It is an object of this invention to propose a new method, system andcomputer program product for determining data transfer margins fornetwork connections which does <sic. do> not have the drawbacksdescribed above. In particular the margins and/or the maximal bit ratesfor a particular user or respectively network point of access should beable to be determined quickly and flexibly without a disproportionaltechnical, personnel and financial investment having to be made.

This object is achieved according to the present invention in particularthrough the elements of the independent claims. Further preferredembodiments follow moreover from the dependent claims and from thedescription.

In particular these objects are achieved through the invention in that,for determining data transfer margins for network connections, thephysical length of a network connection to be determined between atransmitter and a receiver being known, a power spectrum is measured bymeans of a power measuring device in dependence upon the transferfrequency for possible modem types and is transmitted onto a datacarrier <of> a calculating unit,

in that with the calculating unit the attenuation is determined fordifferent physical lengths and cable wire thicknesses of a networkconnection, and the actual signal strengths at the receiver, based onthe attenuation as well as the power spectrum, are stored, assigned tothe respective physical lengths and cable wire thicknesses (i.e. thewire diameters in the cable), in a first list on a data carrier of thecalculating unit,

in that in a second list the noise level is stored, assigned to therespective physical lengths and cable wire thicknesses of the networkconnection, on a data carrier of the calculating unit, the noise levelbeing determined by means of the calculating unit, on the basis of thepower spectrum, depending upon at least crosstalk parameters and numberof interference sources,

in that, by means of a Gaussian transformation module, the calculatingunit determines data transfer margins for a predefined bit rate based onthe actual signal strengths of the first and the corresponding noiselevel of the second list for different data transmission modulationsand/or modulation codings, and stores them, assigned to the respectivephysical lengths and cable wire thicknesses of the network connection,on a data carrier of the calculating unit,

in that the calculating unit determines, based on the stored datatransfer margins, the actual data transfer margins, by means of at leastone or more correction factors, and stores them, assigned to therespective physical lengths and cable wire thicknesses of the networkconnection, on a data carrier of the calculating unit, the correctionfactor comprising an average deviation of the stored data transfermargins with respect to the actual data transfer margins and/or anequalizer factor for correction of the equalizer adjustment, and

in that, based on the stored actual data transfer margins, withreference to the known physical length of the network connection to bedetermined between the transmitter and the receiver, the calculatingunit determines the data transfer margin for the respective networkconnection.

One advantage of the invention is, among other things, that the methodand system permits for the first time a simple and quick determinationof data transfer margins, without having to thereby engage in an immensetechnical investment, investment with respect to personnel andinvestment with respect to time. In particular, the uncertainties can becorrected by means of the mentioned correction, without, as with theremote measuring systems for measuring the data transfer margins and/orthe bit rates, a different imprecisely known uncertainty at each localdistribution station, or respectively unknown errors in measurementhaving to be corrected, which error is difficult to estimate owing tothe single-endedness since measurements on both sides would be necessaryfor determining the error. As described above, the investment remainssmall, compared to the state of the art. This applies both to carryingout the measurement and to installing the necessary devices.

In an embodiment variant, the power spectrum is measured in dependenceupon the transmission frequency for ADSL- and/or SDSL- and/or HDSL-and/or and/or <sic.> VDSL- modem types. The possible SDSL modem typescan thereby comprise at least one G.991.2 modem type and/or the ADSLmodem types at least one G.992.2 modem type. By means of the Gaussiantransformation module, the data transfer margins can be determined forat least the data transmission modulations 2B1Q (2 Binary, 1 Quaternary)and/or CAP (Carrierless Amplitude/Phase Modulation) and/or DMT (DiscreteMultitone) and/or PAM (Pulse Amplitude Modulation). Also by means of theGaussian transformation module, the data transfer margins can bedetermined for at least the trellis modulation coding. This embodimentvariant has, among other things, the advantage that with the xDSL modemtypes, the mentioned data transmission modulations and the trellismodulation coding, common standard technologies are used which areeasily obtainable on the market and whose use are <sic. is> widespreadboth in Europe and also in the USA.

In another embodiment variant, the correction factor reflects anon-linear dependency with respect to the physical lengths and/or cablewire thicknesses, i.e. the correction factor can be represented by anon-linear function, e.g. a polynomial function of an order higherthan 1. This embodiment variant has the advantage, among other things,that much more complex dependencies can be taken into account andcorrected with it than with linear correction factors.

An again different embodiment variant comprises a computer programproduct, which can be loaded directly into the internal memory of adigital computer and comprises software code sections with which thesteps according to the preceding embodiment variants can be carried outwhen the product runs on a computer. This embodiment variant has theadvantage that it makes possible a technical achievement of theinvention which is simple to manage and use without big installations.

In particular, for determining bit rates for network connections, thephysical length of a network connection between a transmitter and areceiver being known, a power spectrum is measured by means of a powermeasuring device in dependence upon the transfer frequency for possiblemodem types, and is transmitted onto a data carrier of a calculatingunit;

with the calculating unit the attenuation is determined for differentphysical lengths and cable wire thicknesses of a network connection, andthe actual signal strengths at the receiver, based on the attenuation aswell as the power spectrum, are stored, assigned to the respectivephysical lengths and cable wire thicknesses, in a first list on a datacarrier of the calculating unit;

in a second list, the noise level is stored, assigned to the respectivephysical lengths and cable wire thicknesses of the network connection,on a data carrier of the calculating unit, the noise level beingdetermined on the basis of the power spectrum by means of thecalculating unit depending upon at least crosstalk parameters and numberof interference sources;

by means of a Gaussian transformation module, based on the actual signalstrengths of the first and the corresponding noise level of the secondlist for different data transmission modulations and/or modulationcodings, the calculating unit determines the bit rates for a predefineddata transfer margin, and stores the bit rates, assigned to therespective physical lengths and cable wire thicknesses of the networkconnection, on a data carrier of the calculating unit;

the calculating unit determines, based on the stored bit rates, theactual bit rates by means of one or more correction factors, and storesthe actual bit rates, assigned to the respective physical lengths andcable wire thicknesses of the network connection, on a data carrier ofthe calculating unit, the correction factor comprising the averagedeviation of the stored bit rates with respect to the actual bit ratesand/or an equalizer factor for correction of the equalizer adjustment;

and based on the stored actual bit rates, with reference to the knownphysical length of the network connection to be determined between thetransmitter and the receiver, the calculating unit determines the bitrate for the respective network connection. This embodiment variant has,among other things, the advantage that the method and system permits forthe first time a simple and quick determination of the bit rates,without having to thereby engage in an immense technical investment,investment with respect to personnel and investment with respect totime. In particular, the uncertainties can be corrected by means of thementioned correction, without, as with the remote measuring systems formeasuring the data transfer margins and/or the bit rates, a differentimprecisely known uncertainty at each local distribution station, orrespectively unknown errors in measurement having to be corrected, whicherrors are difficult to estimate owing to the single-endedness sincemeasurements on both sides would be necessary for determining the error.

In an embodiment variant, the power spectrum is measured in dependenceupon the transmission frequency for ADSL and/or SDSL and/or HDSL and/orand/or <sic . . .> VDSL modem types. The possible SDSL modem types canthereby comprise at least one G.991.2 modem type and/or the ADSL modemtypes at least one G.992.2 modem type. By means of the Gaussiantransformation module, the data transfer margins can be determined forat least the data transmission modulations 2B1Q and/or CAP and/or DMTand/or PAM. Also by means of the Gaussian transformation module, thedata transfer margins can be determined for at least the trellismodulation coding. This embodiment variant has, among other things, theadvantage that with the xDSL modem types, the mentioned datatransmission modulations and the trellis modulation coding, commonstandard technologies are used which are easily obtainable on the marketand whose use are <sic. is> widespread both in Europe and also in theUSA.

In another embodiment variant, the correction factor comprises anon-linear dependency with respect to the physical lengths and/or cablewire thicknesses, i.e. the correction factor can be represented by anon-linear function, e.g. a polynomial function of an order higherthan 1. This embodiment variant has the advantage, among other things,that much more complex dependencies can be taken into account andcorrected than with linear correction factors.

In a further embodiment variant, by means of the Gaussian transformationmodule, the bit rates are determined for data transfer margins between 3and 9 dB. This embodiment variant has the advantage, among other things,that the range between 3 and 9 dB allows reception with QoS parameterssatisfying most demands. In particular the range of the data transfermargins between 3 and 9 dB permits an optimization of the bit rates withrespect to the other QoS parameters.

In a further embodiment variant, by means of the Gaussian transformationmodule, the bit rates for a 6 dB data transfer margin are determined.This embodiment variant has, among other things, the same advantages asthe preceding embodiment variant. In particular, as above, a datatransfer margin of 6 dB permits an optimization of the bit rate withrespect to the other QoS parameters.

An again different embodiment variant comprises a computer programproduct, which can be loaded directly into the internal memory of adigital computer, and comprises software code sections with which thesteps according to the preceding embodiment variants can be carried outwhen the product runs on a computer. This embodiment variant has theadvantage that it makes possible a technical achievement of theinvention which is simple to manage and use without big installations.

It should be stated here that, besides the method according to theinvention, the present invention also relates to a system and a computerprogram product for carrying out this method.

Embodiment variants of the present invention will be described in thefollowing with reference to examples. The examples of the embodimentsare illustrated by the following attached figures:

FIG. 1 shows a block diagram, showing schematically the architecture ofan embodiment variant of a system according to the invention fordetermining data transfer margins or respectively bit rates for anetwork connection 12 with a predetermined physical length 13 between atransmitter 10 and a receiver 11.

FIG. 2 shows schematically a crosstalk interaction with near-endcrosstalk (Next) 51, which describes the undesired coupling of thesignal 50 of the transmitter 10 at one end to the signals 50 at thereceiver 11 at the same end, and far-end crosstalk (FEXT) 52, whichdescribes the undesired coupling of signals 50 during the transmissionto the receiver 11 at the other end, the signals 50 being coupled duringthe transmission to signals 50 of adjacent copper wire pairs and turningup at the receiver 11 as noise.

FIG. 3 shows schematically the transmission distance of the networkconnection in dependence upon the transmission rates (bit rates) forADSL modems, as can be obtained with the system according to theinvention. The reference numerals 60 and 61 thereby designate differentnoise environments.

FIG. 4 shows schematically the so-called last mile of the publictelephone network (PSTN: Public Switched Telephone Network), as existstypically between the end user at home and a network which is supposedto be reached via the public telephone network.

FIG. 1 illustrates an architecture which can be used to achieve theinvention. In this embodiment example for the method and the system fordetermining data transfer margins and/or bit rates for networkconnections, the physical length 13 of a network connection 12 to bedetermined between a transmitter 10 and a receiver 11 is known. Meant bythe physical length is the actual cable length, i.e. not e.g. thedistance by air, between the transmitter 10 and the receiver 11. Thenetwork connection 12 should consist of an analog medium, such as e.g. acopper wire cabling. Used in this embodiment examples were, forinstance, copper cable with 0.4 or 0.6 mm wire diameter, as is typicallyused in the last mile of the public telephone network (PSTN: PublicSwitched Telephone Network). The last mile is shown schematically inFIG. 4. The reference numeral 70 thereby designates a router to anetwork, which is connected via e.g. a 10 BT Ethernet 77 and the publictelephone network (PSTN) 72 to a server 71 with a modem terminal. Themodem terminal server 71 <can> be a DSL Access Multiplexer (DSLAM). Asmentioned, the reference numeral 72 is the public telephone network(PSTN), to which the modem terminal server 71 is connected, for instancevia a fiber optic cable 78. Furthermore the public telephone network 79<sic. 72> or respectively the modem terminal server 71 is connectedtypically via a copper wire cable 79 and via the telephone box 73 to amodem 74 of a personal computer (PC) 75. The reference numeral 79 isthereby the mentioned so-called “last mile” from the distributionstation of the telephone network operator to the end user. The end user76 can thereby access the router 70 directly with his PC by means of thedescribed connection. The common telephone lines of copper can consiste.g. of 2-2400 pairs of copper wires. Other analog media, in particularcopper cable with e.g. other wire diameters, are also conceivable,however. It must be explicitly pointed out that not only can the networkconnections 12 each have different diameters or thicknesses 114, 142,143, 144, but an individual network connection can consist of acombination of cables with different wire diameters or thicknesses, i.e.the network connection can comprise a plurality of sections of cableswith different wire thickness.

A power spectrum PSD_(Modem)(ƒ) is measured in dependence upon thetransmission frequency ƒ for possible modem types 101, 102, 103, 104 bymeans of power measuring device 20, and is transmitted onto a datacarrier of a calculating unit 30. The power spectrum is also designatedas the Power Spectral Density (PSD), and reflects, for a particularbandwidth of a continuous frequency spectrum, the total energy of theparticular frequency bandwidth divided by the particular bandwidth. Thedivision by the bandwidth corresponds to a scaling. The PSD is thus afunction in dependence upon the frequency ƒ, and is normally indicatedin watt per hertz. For power measurement by means of power measuringdevice 20 at the receiver 11, a simple A/D converter can be used, forinstance, the voltage being applied via a resistor. For modulation ofdigital signals to the line 12 e.g. from end user to the distributionstation of the telephone network operator and vice-versa, the mostvarious types of modem can be used. Known in the state of the art aree.g. the xDSL technologies (Digital Subscriber Line), the two mainrepresentatives of which are ADSL (Asymmetric Digital Subscriber Line)and SDSL (Symmetric Digital Subscriber Line). Further representatives ofthe xDSL technology are HDSL (High-data-rate DSL) and VDSL (Very highspeed Digital Subscriber Line). The xDSL technologies are highlydeveloped modulation schemes for modulating data on copper lines orother analog media. xDSL technologies are sometimes also referred to as“last mile technologies,” precisely because they usually serve thepurpose of connecting the last telephone network distribution station tothe end user at the office or at home, and are not used between theindividual telephone network distribution stations. xDSL is similar toISDN (Integrated Services Digital Network) insofar as it can operateover the existing copper lines, and both require a relatively shortdistance to the next distribution station of the telephone networkoperator. xDSL offers however much higher transmission rates than ISDN.xDSL reaches data transmission rates of up to 32 Mbps (bps: bits persecond) downstream rate (transmission rate during reception of data,i.e. during the modulation) and of 32 kbps to 6 Mbps upstream rate(transmission rate during transmission of data, i.e. during thedemodulation), whereas ISDN per channel supports data transmission ratesof 64 kbps. ADSL is a technology which has become very popular recentlyfor modulating data over copper lines. ADSL supports data transmissionrates of 0 to 9 Mbps downstream rate and 0 to 800 kbps upstream rate.ADSL means asymmetrical DSL, since it supports different downstream andupstream rates. SDSL or symmetrical DSL is called symmetrical, on theother hand, because it supports the same downstream and upstream rates.SDSL permits transmission of data up to 2.3 Mbps. ADSL transmits digitalimpulses in a high frequency region of the copper cable. Since thesehigh frequencies are not used in normal sound transmission in theacoustic range, (e.g. voices), ADSL can work at the same time, forinstance, to transmit telephone conversations over the same coppercables. ADSL is widespread in North America, while SDSL was developedabove all in Europe. ADSL as well as SDSL require modems especiallyequipped therefor. HDSL is a representative of symmetrical DSL (SDSL).The standard for symmetrical HDSL (SDSL) is at present G.SHDSL, known asG.991.2, as developed as an international standard of the CCITT (ComitéConsulatif International Téléphonique et Télégraphique) of the ITU(International Telecommunication Union). G.991.2 supports the receptionand transmission of symmetrical data streams over a simple copper wirepair with transfer rates between 192 kbps and 2.31 Mbps. G.991.2 wasdeveloped such that it comprises the features of ADSL and SDSL, andsupports standard protocols such as IP (Internet Protocol), inparticular the current versions IPv4 and IPv6 or IPng of the IETF(Internet Engineering Task Force) as well as TCP/IP (Transport ControlProtocol), ATM (Asynchronous Transfer Mode), T1, E1 and ISDN. To bementioned here as the last of the xDSL technologies is VDSL (Very highspeed Digital Subscriber Line). VDSL transmits data in the range of13-55 Mbps over short distances (usually between 300-1500 m) via twistedpair copper cable. With VDSL it applies that the shorter the distance,the higher the transmission rate. As the final part of a network, VDSLconnects the office or the home of a user to an adjacent optical networkunit, called Optical Network Unit (ONU), which is typically connected tothe main optical fiber network (Backbone), for instance of a company.VDSL allows the user access to the network with maximal bandwidth vianormal telephone lines. The VDSL standard has not yet been fullyestablished. Thus there are VDSL technologies having a Line CodingSchema based on DMT (Discrete Multitone), DMT being a Multi-CarrierSystem having great similarity to the ADSL technology. Other VDSLtechnologies have a Line Coding Schema based on Quadrature AmplitudeModulation (QAM), which, in contrast to DMT, is cheaper, and requiresless energy. For this embodiment example the modem types can compriseADSL and/or SDSL and/or HDSL and/or and/or <sic.> VDSL modem types (101,102, 103, 104). In particular the possible SDSL modem types (101, 102,103, 104) can include at least one G.991.2 modem type and/or the ADSLmodem types (101, 102, 103, 104) at least one G.992.2 modem type. It isclear, however, that this enumeration is not supposed to apply in anylimiting way to the scope of protection of the invention, but that, onthe contrary, other modem types are conceivable.

With the calculating unit 30, the attenuation H is determined fordifferent physical lengths 13 and core thicknesses of the cable 141,142, 143, 144, such as e.g. 0.4 mm and 0.6 mm, of a network connection12, and the actual signal strengths S(ƒ) at the receiver 11, based onthe attenuation H(ƒ) as well as the power spectrum PSD(ƒ), are stored,assigned to the respective physical lengths L 13 and cable wirethicknesses D 141, 142, 143, 144, in a first list on a data carrier ofthe calculating unit 30. Like the actual signal strength S(ƒ), theattenuation H(ƒ,L,D) is thereby a function in dependence upon thefrequency ƒ. The signal sent from the transmitter 10 is thusPSD_(Modem)(ƒ), while at the receiver an actual signal strengthS(ƒ)=PSD_(Modem)(ƒ)H²(ƒ,L,D) is still obtained. In a second list, thenoise level N(ƒ) 40 is stored, assigned to the respective physicallengths 13 and cable wire thicknesses 141, 142, 143, 144 of the networkconnection 12, on a data carrier of the calculating unit 30, the noiselevel N(ƒ) 40 being determined, based on the power spectrum PSD, bymeans of the calculating unit 30, in dependence upon at least crosstalkparameters Xtalk type and number of interference sources A. I.e.

${N(f)} = {\sum\limits_{i,{Xtalktype}}^{\;}{{{PSD}_{{SModern}{(i)}}(f)}{{Hxp}\left( {f,L,{Xtalktype},A_{i}} \right)}}}$

The sum, with the index i, runs over all unwanted modulations (SModem)in dependence upon their Xtalk type, which act on parallel connectionsof the network connection. PSD_(sModem(i)) is the power spectrum of thei^(th) Smodem. Hxp is the attenuation in dependence upon the crosstalk.As mentioned, the crosstalk problem is the physical phenomenon occurringwith modulation of data over a copper cable. Adjacent copper cable wiresinside a copper cable obtain, by way of electromagnetic interaction,partial signals pairwise which are generated by modems. This leads toxDSL modems, which are carried assigned on adjacent wires, interferingwith one another. Crosstalk as the physical effect is almost negligiblefor ISDN (frequency range up to 120 kHz), but becomes important howeverfor e.g. ADSL (frequency range up to 1 MHz) and becomes a decisivefactor for VDSL (frequency range up to 12 MHz). As described, theconventional telephone copper lines consist of 2 to 2400 copper wires.In order to be able to use four pairs, for example, the data stream atthe transmitter is divided up into a multiplicity of parallel datastreams and recombined again at the receiver, which increases the actualdata throughput by a factor of 4. This would permit a data transmissionwith up to 100 Mbps. In addition, in the case of 4 pairs of copperwires, the same four pairs of wire could be used to transport the samequantity of data simultaneously in the opposite direction. Thebidirectional data transmission over each pair of copper wire doublesthe information capacity which can be transmitted. This increases inthis case the data transmission rate by eight times compared toconventional transmissions, in which two pairs are used for onedirection in each case. For data transmission as described above,crosstalk noise is a greatly limiting factor. As crosstalk types adistinction is made between near-end crosstalk (Next) 51, whichdescribes the undesired coupling of the signal 50 of the transmitter 10at one end to the signals 50 at the receiver 11 at the same end, andfar-end crosstalk (FEXT) 52, which describes the undesired coupling ofsignals 50 during the transmission to the receiver 11 at the other end,the signals 50 being coupled during the transmission to signals 50 ofadjacent copper wire pairs and turning up at the receiver 11 as noise(see FIG. 1). Normally it is assumed that NEXT 51 has only one near-endinterference source. Xtalk type is thus dependent upon the location andthe stream (up/down), i.e. Xtalk type (stream, location). If there aremore than two copper wires, which is usually the case (typically thereare between 2 and 2400 wires), then the pairwise coupling describedabove is no longer true. E.g. for the case where four pairs of wire areused at the same time, there are consequently now three unwantedinterference sources which couple with their energy to the signal 50.For A, A=3 applies in this case. The same applies for FEXT crosstalk 52.

By means of a Gaussian transformation module 31, the calculating unit 30determines the data transfer margins based on the actual signal strengthstrengths S(ƒ) of the first and the corresponding noise level R(ƒ) ofthe second list for different data transmission modulations and/ormodulation codings for a predefined bit rate, and stores the datatransfer margins, assigned to the respective physical lengths 13 andcable wire thicknesses 141, 142, 143, 144 of the network connection 12,on a data carrier of the calculating unit 30. With the actual signalstrengths S(ƒ) of the first list and the noise level N(ƒ), the signal Sto noise R <sic. N> ratio SNR (Signal to Noise Ratio) can be calculatedby means of the calculating unit 30, whereby:

${SNR} \cong {\exp\left( {T{\int_{{{- 1}/2}T}^{{1/2}T}{{\ln\left( \frac{\sum\limits_{n}{{S\left( {f + {n/T}} \right)}}^{2}}{\sum\limits_{n}{N\left( {f + {n/T}} \right)}} \right)}{\mathbb{d}f}}}} \right)}$

This formula applies only for CAP, 2B1Q and PAM modulation, not howeverfor DMT modulation. DMT will be described more closely further below. Tis thereby the symbol interval or half the inverse of the Nyquistfrequency. The Nyquist frequency is the highest possible frequency thatcan still be sampled precisely. The Nyquist frequency is half thesampling frequency, since unwanted frequencies are generated when asignal is sampled whose frequency is higher than half the samplingfrequency n is the summing up index. In practice it normally sufficesfor n to run from −1 to 1. If this does not suffice, further maxima 0,±1/T, ±2/T etc. can be included until the desired precision is reached.The data transfer margins depend upon the data transmission modulationsand/or modulation codings, as has been mentioned further above. In thisembodiment example we shall show the dependency, for instance, for HDSLmodems 2B1Q modulation (2 Binary, 1 Quaternary) and CAP modulation(Carrierless Amplitude/Phase Modulation) as an example for ADSL DMTmodulation (Discrete Multitone Technology) and with respect to themodulation codings for trellis-coded signals. However, it is also clearthat the method and system according to the invention also applies,without further ado, to other data transmission modulations and/ormodulation codings such as e.g. PAM (Pulse Amplitude Modulation) etc.2B1Q modulation as well as CAP modulation is used with HDSL modems, andhas a predefined bit rate. DMT modulation is used with ADSL modems, andhas, on the other hand, a variable bit rate. CAP and DMT have used thesame fundamental modulation technology: QAM (Quadrature AmplitudeModulation), although this technology is employed differently. QAM makesit possible for two digital carrier signals to occupy the sametransmission bandwidth. Two independent so-called message signals arethereby used to modulate two carrier signals having an identicalfrequency, but differing in amplitude and phase. QAM receivers candistinguish whether a low or a high number of amplitude and phase statesare required in order to obviate noise and interference e.g. on a copperwire pair. 2B1Q modulation is also known as “4 Level Pulse AmplitudeModulation” (PAM). It uses two volt levels for the signal pulse and not,such as e.g. AMI (Alternate Mark Insertion), one level. Since positiveand negative level distinction is also made, one obtains a 4 levelsignal. The bits are combined finally into twos in each case, whichpairs each correspond to a volt level (therefore 2 bit). The requiredsignal frequency for transmitting the same bit rate, as with bipolarAMI, is thereby halved with 2B1Q. With HDSL modem with 2B1Q or CAPmodulation, there exists the following dependency of the data transfermargins with respect to the SNR:M _(c) =SNR/ξ

whereby ξ can be determined as a function of the error rate (SymbolError Rate) ε_(s). For LAN (IP) an error rate of ε_(s)=10⁻⁷ usuallysuffices, i.e. each 10⁷ bit is wrongly transmitted on the average.Companies typically require a ε_(s)=10⁻¹² for their company networks.If, for instance, the ε_(s) approaches the order of magnitude of thedata packet size transmitted (e.g. 10⁻³), that would mean converselythat each packet has to be transmitted twice on the average until itarrive correctly. For the 2B1Q modulation there applies for ε_(s) forexample:

$ɛ_{s} = {2{\left( {1 - \frac{1}{M}} \right) \cdot {G_{c}\left( \sqrt{\frac{3*\xi}{M^{2} - 1}} \right)}}}$for uncoded signals and

$ɛ_{s} = {2{\left( {1 - \frac{1}{M/2}} \right) \cdot {G_{c}\left( \sqrt{\frac{3*\xi*10^{0.4}}{\left( {M/2} \right)^{2} - 1}} \right)}}}$for trellis-coded signals, while for the CAP modulation there applies:

$ɛ_{s} = {4{\left( {1 - \frac{1}{M}} \right) \cdot {G_{c}\left( \sqrt{\frac{3\xi}{M^{2} - 1}} \right)}}}$for uncoded signals and

$ɛ_{s} = {4{\left( {1 - \frac{1}{M/\sqrt{2}}} \right) \cdot {G_{c}\left( \sqrt{\frac{3\left( {\xi 10}^{0.4} \right)}{{M^{2}/2} - 1}} \right)}}}$for trellis-coded signals. for both codings G_(c) is a complementaryGauss function with:

${G_{c}(x)}:={\int_{x}^{\infty}{\frac{1}{\sqrt{2\pi}}{\mathbb{e}}^{{- x^{\prime 2}}/2}{\mathbb{d}x^{\prime}}}}$and for the 2B1Q modulation M is the moment number with M=4 for 2B1Q,while for the CAP modulation M is the constellation magnitude M×M. T is,as above, the symbol interval or half the inverse of the Nyquistfrequency. For ADSL modems with DMT modulation, the dependency isdifferent. As mentioned, ADSL has a variable bit rate. This displaysitself likewise in M_(c). Applicable is:

$M_{c} = {x_{ref}\frac{{2{\left( {\int{{\log_{2}\left( {1 + \frac{\xi(f)}{x_{ref}\Gamma}} \right)}{\mathbb{d}f}}} \right)/\Delta}\; f} - 1}{2^{{D/\Delta}\; f} - 1}}$whereby ξ(ƒ) is the signal-to-noise ration S(ƒ)/N(ƒ). x_(ref) is areference margin which in this embodiment example has been typicallyselected as 6 dB, i. e. x_(ref)=10^(0.6). Other values for referencemargins are conceivable, however. Δƒ is the entire frequency width orrespectively the entire frequency band used for the transmission. Theintegration is executed via the frequency. D is the bit rate, forinstance in b/s (bits/seconds). Γ is a correction factor. In thisembodiment example Γ is situated for instance at Γ=9.55. The integrationis carried out in this embodiment example via the frequency ƒ.Analogously, it can also be carried out over time or another physicalvalue, the formula above having to then be adapted accordingly.

In general, the data transfer margins obtained such as above do notcorrespond to experiment. Therefore the calculating unit 30 determinesthe actual data transfer margins by means of at least one correctionfactor based on the stored data transfer margins. The correction factorfor this embodiment example has been selected such that a sufficientcorrespondence is achieved between the obtained data transfer marginsand the actual data transfer margins. Assumed to be sufficient here wase.g. +/−3 dB, other values also being conceivable, however. To achievethis maximal deviation of +/−3 dB, two parameters are determined.M_(imp) takes into account the good or poor implementation of a modem bythe manufacturer. M_(imp) was introduced based on the fact that samemodems with comparable hardware and same data transmission modulationsand /or modulation codings, but however from different manufacturers,deliver different results during translation of the analog signal into adigital signal and vice-versa, which affects their maximal bit rate ortheir maximal range for a particular network connection. This must becorrected for the data transfer margins. Introduced as the secondparameter was N_(int). N_(int) takes into account the quantization noisein the modem (analog-to digital conversion), as well as a possible pooradaptation of the equalizer during the transmission. If a transmissiontakes place between a transmitter 10 and a receiver 11, the equalizer inthe modem adapts the transmission rate to the conditions of the networkconnection such as e.g. the line attenuation, phase distortion, etc. bymeans of a training sequence, which are <sic. is> sent back and forthbetween the two communicating modems. A poor adaptation by the equalizerleads to a distortion of the results and must be corrected. For linearequalizers, the following formula can be used, for example:

${SNR}_{LinearEq} = {\left( {T{\int_{{{- 1}/2}T}^{{1/2}T}\frac{\mathbb{d}f}{X_{s}(f)}}} \right)^{- 1}\mspace{20mu}{with}}$${X_{s}(f)} = {{\sum\limits_{n}\frac{{{S_{e}\left( {f + {n/T}} \right)}}^{2}}{N_{e}\left( {f + {n/T}} \right)}} + 1}$

whereby SNR_(linearEq) is the signal-to-noise ratio, S_(e) the signalwhich the equalizer receives, N_(e) the noise and ƒ the frequency. For aDecision Feedback Equalizer (DFE), the following formula can be used:

SNR_(DFE) = exp (T∫_(−1/2T)^(1/2T)ln (X_(s)(f))𝕕f)  with${X_{s}(f)} = {{\sum\limits_{n}\frac{{{S_{e}\left( {f + {n/T}} \right)}}^{2}}{N_{e}\left( {f + {n/T}} \right)}} + 1}$

whereby again SNR_(linearEq) is the signal-to-noise ratio, S_(e) is, asabove, the signal which the equalizer receives, N_(e) the noise and ƒthe frequency. For determination of the SNR_(DFE), the calculating unit30 can use e.g. the following approximation:

${SNR}_{DFE} \cong {\exp\left( {T{\int_{{{- 1}/2}T}^{{1/2}T}{{\ln\left( \frac{\sum\limits_{n}{{S_{e}\left( {f + {n/T}} \right.}^{2}}}{\sum\limits_{n}{N_{e}\left( {f + {n/T}} \right)}} \right)}{\mathbb{d}f}}}} \right)}$

Thus it follows for the actual data margins:S(ƒ)=PSD_(Modem)(ƒ)H²(ƒ,L,D) as previously. The noise is corrected asfollows:

${N(f)} = {{\sum\limits_{i}{{{PSD}_{{SModem}{(i)}}(f)} \cdot {{Hxp}^{2}\left( {f,L,D,{xtalktype}_{i},n_{i}} \right)}}} + N_{int}}$

In the calculating unit 30 the correction can be implemented in a moduleusing hardware or software. It is important to point out that with sucha module, based on the correction N_(int), a variable noise factor isintroduced which can take into consideration, for example, equalizerharmonization, etc. This cannot be found as such in the state of theart, and is among the substantial advantages of the invention, amongother things. The actual data transfer margins M_(eff) become <have beengiven> through M_(eff)=M_(c)−M_(imp), which is taken into account inaddition to N_(int), as mentioned above. The correct values for M_(c)and N_(int) can be obtained by the calculating unit 30 in the comparisonwith experimental data. Typically the calculating unit 30 must haveaccess for this purpose to data from various experiments in order to beable to determine the parameters correctly within the desired deviation.By means of the correction factors, which therefore comprise an averagedeviation of the stored data transfer margins with respect to the actualdata transfer margins, the actual data transfer margins described aboveare determined and stored, likewise assigned to the respective physicallengths L 13 and cable wire thicknesses D 141, 142, 143, 144 of thenetwork connection 12, on a data carrier of the calculating unit 30. Itis to be pointed out that the correction factors do not necessarily haveto be linear factors, i.e. constants, but can also just as well compriseinstead correction functions with a non-linear dependency. Dependingupon the application, more complex deviations of the experimental datacan thereby also be taken into account. Finally, by means of the storedmatrices with the data transfer margins, the calculating unit 30determines the data transfer margin for a particular network connection12 based on the stored actual transfer margins with reference to theknown physical length 13 of the network connection 12 to be determinedbetween the transmitter 10 and the receiver 11. As mentioned severaltimes, the data transfer margins are indicated in dB. The modem runstypically for values >0 dB, while for values <0 dB it does not run. Toguarantee a good, secure operation, it can make sense to select e.g. 6dB as lower limit. In general, other data transfer margins are alsosuitable as lower limit, however, e.g. values between 3 dB and 9 dB. Asfollows from the above indications, instead of matrices with datatransfer margins, correspondingly matrices with bit rates for variousnetwork connections, e.g. for a data transfer margin of 6 dB, can bedetermined for ADSL modems, by means of the same configuration. Thus itfollows for determining the matrices with bit rates 6 dB=M_(eff). In thecase of the HDSL modems, this does not make any sense insofar as thecodings with HDSL, such as e.g. 2B1Q or CAP, work with a constant bitrate, here e.g. 2.048 Mb/s. The reason for this difference with respectto the ADSL modems is that HDSL systems are only designed for a point ofaccess with higher bit rate, and concern only security (SNR).

FIG. 3 shows the transmission distance of the network connection independence upon the transmission rate (bit rate) for ADSL modems. Thereference numerals 60 and 61 thereby designate different noiseenvironments. As described above, the bit rates have been shown based onthe stored matrices or respectively lists.

1. A method for determining data transfer margins for networkconnections, a physical length of a network connection to be determinedbetween a transmitter and a receiver being known, comprising: measuringa power spectrum by a power measuring device in dependence upon atransfer frequency for possible modem types, and transmitting themeasured power spectrum onto a data carrier of a calculating unit;determining, with the calculating unit, attenuation for differentphysical lengths and cable wire thicknesses of the network connection,and storing actual signal strengths at the receiver, based on theattenuation and the measured power spectrum, assigned to the respectivephysical lengths and cable wire thicknesses, into a first list on thedata carrier of the calculating unit; storing, in a second list, noiselevel, assigned to the respective physical lengths and cable wirethicknesses of the network connection, on the data carrier of thecalculating unit, the noise level being determined by the calculatingunit based on the power spectrum, depending upon at least crosstalkparameters and number of interference sources; determining, by aGaussian transformation module, in the calculating unit, data transfermargins for a predefined bit rate based on the actual signal strengthsof the first list and corresponding noise level of the second list fordifferent data transmission modulations and/or modulation codings, andstoring the data transfer margins, assigned to the respective physicallengths and cable wire thicknesses of the network connection, on thedata carrier of the calculating unit; determining, in the calculatingunit, based on the stored data transfer margins, the actual datatransfer margins by at least one correction factor and storing theactual data transfer margins, assigned to the respective physicallengths and cable wire thicknesses of the network connection, on thedata carrier of the calculating unit, the correction factor comprisingan average deviation of the stored data transfer margins with respect tothe actual data transfer margins and/or an equalizer factor forcorrection of equalizer adjustment; and determining, based on the storedactual data transfer margins, with reference to the known physicallength of the network connection to be determined between thetransmitter and the receiver, in the calculating unit, the data transfermargin for the respective network connection.
 2. A method according toclaim 1, wherein the correction factor reflects a non-linear dependencywith respect to the physical lengths and/or cable wire thicknesses.
 3. Amethod according to claim 1, wherein the power spectrum is measured independence upon transmission frequency for ADSL, and/or SDSL, and/orHDSL, and/or VDSL modem types.
 4. A method according to claim 3, whereinthe SDSL modem types comprise at least one G.991.2 modem type, and/orthe ADSL modem types comprise at least one G.992.2 modem type.
 5. Amethod according to claim 1, wherein by the Gaussian transformationmodule the data transfer margins are calculated for at least the datatransmission modulations 2B1Q, and/or CAP, and/or DMT, and/or PAM.
 6. Amethod according to claim 1, wherein the data transfer margins aredetermined for at least trellis modulation coding by the Gaussiantransformation module.
 7. A computer program product, configured to beloaded directly into an internal memory of a digital computer andcomprising software code sections with which operations according toclaim 1 are able to be executed when the product runs on a computer. 8.A method for determining bit rates for network connections, a physicallength of a network connection between a transmitter and a receiverbeing known, comprising: measuring a power spectrum by a power measuringdevice in dependence upon transfer frequency for possible modem typesand transmitting the measured power spectrum onto a data carrier of acalculating unit; determining, with the calculating unit, attenuationfor different physical lengths and cable wire thicknesses of the networkconnection, and storing actual signal strengths at the receiver, basedon the attenuation and the measured power spectrum, assigned to therespective physical lengths and cable wire thicknesses, in a first liston the data carrier of the calculating unit; storing, in a second list,noise level, assigned to the respective physical lengths and cable wirethicknesses of the network connection, on the data carrier of thecalculating unit, the noise level being determined based on the measuredpower spectrum by the calculating unit depending upon at least crosstalkparameters and number of interference sources; determining, by aGaussian transformation module, based on the actual signal strengths ofthe first list and corresponding noise level of the second list fordifferent data transmission modulations and/or modulation codings, inthe calculating unit, bit rates for a predefined data transfer margin,and storing the bit rates, assigned to the respective physical lengthsand cable wire thicknesses of the network connection, on the datacarrier of the calculating unit; determining, in the calculating unit,based on the stored bit rates, the actual bit rates by a correctionfactor, and storing the actual bit rates, assigned to the respectivephysical lengths and cable wire thicknesses of the network connection,on the data carrier of the calculating unit, the correction factorcomprising an average deviation of the stored bit rates with respect tothe actual bit rates and/or an equalizer factor for correction ofequalizer adjustment; and determining, based on the stored actual bitrates, with reference to the known physical length of the networkconnection to be determined between the transmitter and the receiver, inthe calculating unit, the bit rate for the respective networkconnection.
 9. A method according to claim 8, wherein by the Gaussiantransformation module the bit rates are determined for a data transfermargin between 3 and 9 dB.
 10. A method according to claim 8, wherein bythe Gaussian transformation module the bit rates are determined for a 6dB data transfer margin.
 11. A method according to claim 8, wherein thecorrection factor reflects a non-linear dependency with respect to thephysical lengths and/or cable wire thicknesses.
 12. A method accordingto claim 8, wherein the power spectrum is measured in dependence upontransmission frequency for ADSL, and/or SDSL, and/or HDSL, and/or VDSLmodem types.
 13. A method according to claim 12, wherein the SDSL modemtypes comprise at least one G.991.2 modem type, and/or the ADSL modemtypes comprise at least one G.992.2 modem type.
 14. A method accordingto claim 8, wherein by the Gaussian transformation module the bit ratesare determined for at least the data transmission modulations 2B1Q,and/or CAP, and/or DMT, and/or PAM.
 15. A method according to claim 8,wherein by the Gaussian transformation module the bit rates aredetermined for at least trellis modulation coding.
 16. A computerprogram product, configured to be loaded directly into an internalmemory of a digital computer and comprising software code sections withwhich the operating according to claim 8 are executed when the productruns on a computer.
 17. A system for determining data transfer marginsfor network connections, a physical length of a network connection to bedetermined between a transmitter and a receiver being known, comprising:a measuring device configured to measure a power spectrum in dependenceupon transmission frequency for possible modem types; a data carrier ofa calculating unit, on which the power spectrum is storable; wherein thecalculating unit comprises means for determining attenuation fordifferent physical lengths and cable wire thicknesses of the networkconnection, the actual signal strengths at the receiver, based on theattenuation as well as the power spectrum, being stored, assigned to therespective physical lengths and cable wire thicknesses, in a first liston the data carrier of the calculating unit; wherein the calculatingunit comprises means for determining noise level, based on the measuredpower spectrum, depending upon at least crosstalk parameters and numberof interference sources, the noise level being stored, assigned to therespective physical lengths and cable wire thicknesses of the networkconnection, in a second list on the data carrier of the calculatingunit; wherein the calculating unit comprises a Gaussian transformationmodule configured to determine data transfer margins for a predefinedbit rate based on the actual signal strengths of the first list andcorresponding noise levels of the second list for different datatransmission modulations and/or modulation codings, the data transfermargins being stored, assigned to the respective physical lengths andcable wire thicknesses of the network connection, on the data carrier ofthe calculating unit; wherein the calculating unit comprises acorrection module configured to determine, based on the stored datatransfer margins, the actual data transfer margins by at least onecorrection factor, and to store the actual data transfer margins,assigned to the respective physical lengths and cable wire thicknessesof the network connection, on the data carrier of the calculating unit,the correction factor comprising an average deviation of the stored datatransfer margins with respect to the actual data transfer margins and/oran equalizer factor for correction of equalizer adjustment.